The present invention relates to a color image forming apparatus represented by a color electronic copying machine, a color printer and the like using an electrophotographic method, which layers single-color images to form a color image.
U.S. Pat. No. 5,087,945 discloses an image forming apparatus which includes four image forming sections and two transfer drums. Color-component images formed by the four image forming sections are layered and outputted to a transfer material by one of the transfer drums.
In the image forming section apparatus disclosed in this U.S. Patent, a first image is formed on one transfer drum. A second image is formed on the same transfer drum at a timing at which the first image is conveyed by rotation of the transfer drum, and thus, first and second images are layered. The layered images are transferred to the other transfer drum. Thereafter, third and fourth images are sequentially formed thereon at timings at which the layered images are conveyed by rotation of the transfer drum, thereby to layer the images on the same transfer drum.
In the image forming apparatus described above, duet to the directions in which two intermediate transfer drums 5 are rotated and the feature of the layout of four image forming units 1 to 4 faced to the drums, for example, an image formed by the first image forming unit 1 is turned once on an intermediate transfer drum 5 which rotates in the clockwise direction, and is thereafter turned substantially three or four times on an intermediate transfer drum 5 which rotates in the anticlockwise direction. The image is then conveyed to a transfer position faced to the roller 7. Therefore, there is a problem that the time until images of four colors are layered and form one color image is elongated.
In addition, since the position of the image forming unit for forming a black (monochrome) image is node clearly defined, there is also a problem that the first copy time required until image formation of the first image after an instruction of the image formation is elongated.
The length of the first copy time is equal both in the case of forming a black image of single color and the case of forming a color image by layering four color images. Particularly when forming one sheet of single black image, the waiting time is long.
In addition, from another proposal not shown in the figures, four photosensitive drums are provided as image carriers in many color image forming apparatuses, and a charger, an exposure device, a developing device, a transfer device, a cleaning device, and a discharger are provided orderly around each of the photosensitive drums. Each of the first to fourth image forming units for forming predetermined colors is constructed by those devices provided along the circumference of the photosensitive drum.
The first to fourth image forming units are a set of total four units, i.e., an image forming unit for yellow, an image forming unit for magenta, an image forming unit for cyan, and an image forming unit for black.
In each of the first to fourth image forming units, a well-known electrostatic photographic process in the order of charge, exposure, development, transfer, cleaning, to discharge is carried out on the surface of the photosensitive drum. Meanwhile, a paper sheet as a transfer material is let pass through the units sequentially, so visible images of predetermined (four) colors formed by the image forming units are transferred one after another. Further, the images of multiple colors are fixed finally by a fixing device not shown which is provided next to the fourth image forming unit.
In this kind of color image forming apparatus, toner images in four colors formed by four image forming units are sequentially layered in case of forming an image of a predetermined color. Also, in this kind of color image forming apparatus, images are superposed at most four times.
Further another proposal for a well-known color image forming apparatus shows a method as follows. Toner images formed by four toner image forming sections for forming images of three colors respectively corresponding to the color components of subtractive primaries and a black image for strengthening the black density are formed on a belt-like photosensitive surface for every color, and are layered on an intermediate transfer drum. Thereafter, images of four colors are transferred onto a paper sheet by a transfer device.
However, in the color image forming apparatus of this kind, the total length of the intermediate drum is set to the maximum length of the image to be printed out, so there is a problem that the diameter is increased.